The ‘time‘ option signals that the datatype is indeed time/date. If the
option is not specified, the datatype reverts to normal.

See timefmt to tell gnuplot how to read date or time data. The
time/date is converted to seconds from start of the century. There is
currently only one timefmt, which implies that all the time/date columns must
conform to this format. Specification of ranges should be supplied as quoted
strings according to this format to avoid interpretation of the time/date as
an expression.

The function ’strftime’ (type "man strftime" on unix to look it up) is used
to print tic-mark labels. ‘gnuplot‘ tries to figure out a reasonable format
for this unless the ‘set format x "string"‘ has supplied something that does
not look like a decimal format (more than one ’%’ or neither %f nor %g).